Field of the Invention
The present invention relates to a process for removing dissolved oxygen from the aqueous media of a steam generation or other system to prevent corrosion of the system.
In various systems, such as in boiler systems for energy production or other heater systems, an aqueous medium is recirculated that eventually picks up unwanted oxygen, the oxygen becoming dissolved in the aqueous medium.
The presence of dissolved oxygen in such an aqueous medium has a corrosive effect on the downstream components of the system, such as in feedwater heaters and steam generators. Often, such oxygen finds its way into the aqueous medium through the condenser contained in such systems, due to the differential pressure that exists between the atmosphere and the subatmospheric pressure inside the condenser. In high pressure boiler systems, oxygen removal has been performed by hydrazine dosing of the feedwater to the boiler.
The use of hydrazine as an oxygen scavenger, for dissolved oxygen in water, is known. The hydrazine is said to combine with dissolved oxygen in such systems according to the equation: EQU N.sub.2 H.sub.4 +O.sub.2 .fwdarw.H.sub.2 +2H.sub.2 O.
In U.S. Pat. No. 3,551,349, the addition of an aqueous hydrazinequinone mixture is suggested for corrosion protection. The addition of chemical activators or catalysts to aqueous hydrazine, in order to increase the rate of removal of dissolved oxygen from the aqueous system, is preferred. As examples of such catalysts, U.S. Pat. No. 3,808,138 teaches the use of a cobalt maleic acid hydrazide with hydrazine; U.S. Pat. No. 3,843,547 teaches the use of a mixture of an arylamine compound and a quinone compound with hydrazine; and U.S. Pat. No. 4,142,500 teaches the use of nitro- or amino-substituted phenols or cresols with hydrazine to increase the rate of dissolved oxygen removal. While in some instances the additive may be designated a catalyst and in other instances as an activator, the purpose of such additives is to improve the rate of removal of dissolved oxygen from systems using hydrazine. This is due to a problem that is associated with use of hydrazine as an oxygen scavenger, in that it has low reaction rates at typical condenser temperatures of about 80.degree.-130.degree. F. In attempting to solve the problem by use of additives, however, there is the further problem of the nature of such additives and associated decomposition products and their impact on steam generator materials.
As described by S. R. M. Ellis and C. Moreland in Proceedings of the International Conference on Hydrazine and Water Treatment (Bournemouth, England), May 15-17, 1957, at pages 17 and 18, ultraviolet light has an accelerating effect on an oxygen-hydrizine reaction. The data presented however was conducted with oxygen saturated water (approximately 8 ppm) and in a sodium phosphate water chemistry.
It is an object of the present invention to utilize the ultraviolet radiation catalyzing effect on hydrazine in a steam generation system at a location which will provide a speedy reaction and remove oxygen from aqueous medium recycled to the steam generator.
It is another object of the present invention to provide an oxygen-removal system using hydrazine which eliminates the need for chemical additives and the subsequent problems associated with such additives and decomposition products of such additives.